Spotlight on Lighting #3 : How much Light? (Watts, Lumens, Lux)

Getting the lighting right is often an after thought when it comes to lighting our clients' homes, and yet the way that a room is illuminated at night can make a huge difference, both to how the room functions as well as to how it feels.

"Light is the major enabler in assisting perceptions of interiors"

Clive Edwards - "Interior Design - a critical introduction" (BTW this is a superb book which I recommend to anybody keen to get an overview of the Interior Design Industry!)

There are three main types of lighting for a room: ambient (the main light in the room), task and decorative (also known as accent lighting).

You can read more about these three types in our first blog post of this series.

The secret to lighting your room well is to use a mixture of all three types of lighting, at different strengths, and in different ways. Layering the lighting in your client's room by using a mixture of pendants / chandeliers, table lamps, floor lamps and under counter lights gives depth and dimension to a room, highlighting key architectural features and setting the mood for the space.

Sounds complicated? It doesn't have to be!

Identify Your Lighting Needs

The type and quantity of lighting you will need will depend upon the purpose of the room. Generally speaking, you will need stronger light in rooms where your client will be completing tasks (such as kitchens), and more gentle light in rooms where your clients are relaxing (a living room for example).

Many of us live in open plan spaces these days, and this means that our lighting needs can change according to how we use a room throughout the day. A dining room table, for example could be used for completing homework one evening but for hosting a dinner party the next evening. Both of these activities require different levels of lighting.

This can be planned for, by ensuring that the light bulbs are strong enough for tasks, and then adding dimmer switches so that the light can be softened for entertaining. Side table lamps can also help to spread the light across the room.

So... How Much Light Do I Need?

In our last blog we looked at how you measure the strength of light emitted by a bulb, by using a measure called lumens. Lumens are displayed on the side of the light bulb packaging. The higher the number of lumens the brighter the bulb will be.  Amazon use Lumens (or lm) as a key filter to aid the customer to get to the correct bulb they desire.  But how many lumens do you need?


Now this isn't as straightforward as I'd like, but stay with me here as I explain.

A bulb of say, 1,000 lumens might look really bright in a very small room, but in a much larger room, the light is going to appear dimmer because there is not enough brilliance to shine across the whole room. Make sense?

So lumens on their own are not enough to tell us how much light we need to consider the size of the room that the bulb will be in too.

To work out how bright we want the room to be we use a measurement called LUX. LUX takes into account the lumens within an area.  1 LUX is equal to the light given off by 1 lumen over 1 square metre.

As discussed above, different rooms and different tasks will need different lighting amounts. And so the first thing we need to do is to decide how bright (in lux) we want our room to be. Back to Amazon briefly and you will notice that Lux is not a filter offered, as each room is a different size.  Lux therefore is something the designer has to work out for themselves. Below is a rough guide to how many lux you want to achieve in different rooms, or for different tasks.

The Ideal Brightness for Different Tasks in Different Rooms

Relaxation rooms (e.g. TV rooms) 120-150 Lux

For reading 200 Lux

Kitchen: 150-200 Lux (ambient), 250-400 Lux (task)

Dining Room: 100-200 Lux (ambient)

Domestic Office: 150 Lux (ambient), 400 Lux (task)

Home Workshop for detailed work: (e.g. sewing) 500-700 Lux

Bedrooms: 100 Lux (ambient), 200-300 Lux (task e.g. reading)

Lux Levels

Our lux levels chart above details the light levels required for lighting the average space, depending on their use. This will help you choose the perfect indoor lighting and lamps for your home or business.

It is important that you get the right lighting levels in a room, as it can cause a number of detrimental effects on a client's mental and physical health. Side effects can include eye strain, headaches, tiredness and fatigue, neck back and shoulder strain and even depression.

What is Lux?

Lux is a measurement of light intensity, commonly referred to as 'illumination'. It measures the amount of light present on a particular surface as measured by the human eye. In essence this is light intensity within a specific area. The purpose of lux is to provide the number of lumens needed to sufficiently light a given space. For example, a sufficiently lit office requires around 400 -500 lux.

What is a Lumen?

This is a common way to identify the strength of LED bulbs, so it's important you understand what it means. A typical LED bulb will have the lumen level indicated on the packaging.

The official measure of illuminance is in Lumen. Lumen is another way to measure the output of light i.e. the brightness. It measures the emitted light per second from a source but doesn't consider the light's direction. Put simply, the higher the lumen, the higher the brightness/output of the light.

One Lumen per square metre equals one lux, so it is easy to convert the two. Unlike Lux, Lumen doesn't consider the direction of the light or the space the light is filling. Therefore, it shouldn't be considered in isolation. Both Lux and Lumen should be considered when selecting lighting.

You may see the occasional reference to 'foot candle', this is a non-SI measurement commonly used in the United States and uses a square foot as the measurement instead of a square metre.  So 1 foot candle is equal to 1 lumen per square foot.

Watts v Lumen v Lux

Watts measures a light source's power output rather than the level of brightness. New, energy-efficient technology now means that using watts isn't an accurate indication of a bulb's power. As technology continues to improve, more lumen will require fewer watts, improving energy efficiency. Do not use watts as an indication of a bulb's brightness, as it simply doesn't give an accurate representation.

The importance of good lighting cannot be overestimated. It is important that you understand what the numbers on the packaging mean, plus which lighting level is suitable for the intended room. A larger number doesn't necessarily mean better especially when considering watts.

So once you know how bright you want the room to be, you just multiply the brightness in LUX by the area of your room, and this gives you the bulb strength you need in lumens.

A great quick reference guide written by Ros Humphries which goes into a little more detail including class symbols etc. can be found on the Habitat website for those of you who want to read a little more on the subject.

Now, Let's look at an example:

A living room is 3m wide by 4m long giving an area of 12 square metres.

Because it is a living room, where you'll be relaxing and watching TV, you probably want the room to have a brightness of around 120 Lux.

So to know how many lumens you need, you just multiply the Lux by the area. So in this example 120 Lux x 12 square metres = 1440 Lumens.

So to light the room you need either one bulb of 1440 Lumens or several bulbs which add up to 1440 Lumens.

If in doubt buy a brighter bulb than you think you need and install a dimmer switch.

Share the Light Out

Once you know how many lumens you need for your space to be lit adequately, you then need to decide how many light fittings (lamps, pendants etc.) you want to use.

You could of course just use one central bulb in a pendant and be done, but this rarely casts the best light around a room. The secret to good lighting is to layer the lumens you need for the room across different heights. So consider using table lamps, floor lamps and wall sconces as well as pendants.

Take the number of Lumens and share them out across your different light fittings.

So if your room requires 1000 Lumens, for example you could use two side lamps, each with a 300 lumens bulb and a floor lamp with 400.

If maths is not something you like to spend any more than the absolute minimum amount of time doing, I have designed a quick spreadsheet which calculates a full lighting plan for you based on some simple inputs.  You can access it here.

You will need to make a copy of the file so that you can enter your data and use the calculator.  Happy lighting...

Until next time...

Kate x

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